Conventional ventilators provide ventilatory support by utilizing a number of different pressure-time profiles. In its simplest form, a ventilator delivers airflow at a fixed rate (or some other fixed function of time such as sinusoidally), and the airway pressure increases passively as a function of the mechanical properties of the patient's respiratory system. Such a ventilator is in general suitable only for a paralyzed and sedated patient who cannot change his/her ventilation at will. Also, the system is intolerant of leak, so is unsuitable for non-invasive (mask) ventilation.
A bilevel ventilator uses a square pressure-time waveform:P=P0+A, f>0P=P0, otherwise
where P0 is an end expiratory pressure, chosen to splint the upper and lower airways and alveoli, A is a fixed pressure modulation amplitude chosen to supply a desired degree of support, and f is respiratory airflow. Here, and throughout what follows, inspiratory flow is defined to be positive, and expiratory flow is defined to be negative. With bilevel support, the patient can breathe as much or as little as he wishes, by using greater or lesser effort, and the system is somewhat less affected by leak. Some known ventilators, for example, the Servo 300 available from Siemens Medical, Iselin, N.J., and the VPAP-ST from ResMed, San Diego Calif., have an adjustment for changing the initial rate of rise of pressure, with the intention of providing a more comfortable waveform by using a slower rate of rise. In such prior art, the clinician selects a particular waveform, but thereafter the waveform does not change, and there is no automatic adjustment of the waveform.
Moving on in complexity, a proportional assist ventilator provides pressure equal to an end expiratory pressure P0 plus a resistance R multiplied by respiratory airflow, plus an elastance E multiplied by the time integral of respiratory airflow:P=P0+Rf+E∫fdt, f>0P=P0+Rf, otherwise(where the integral is from the time of start of the current inspiration to the current moment) in which the resistance R is chosen to unload some or all of the resistive work of breathing, and the elastance E is chosen to unload some or all of the elastic work of breathing (that is to say, the Rf term provides a pressure increment to offset some or all of the effort required to get air to flow through the mechanical passageways, and the integral term provides some or all of the pressure required to overcome the elastic recoil or springiness of the lungs and chest wall). A proportional assist ventilator amplifies patient effort, delivering a natural-feeling waveform, and it is easier for the patient to increase or decrease his ventilation than in the case of bilevel support. However, a proportional assist ventilator is disadvantageous for a patient with abnormal chemoreflexes, as inadequate support is provided during pathological reductions in effort such as central apneas and hypopneas.
Another approach is to provide a pressure-time profile that is continuous function of phase in the respiratory cycle:P=P0+AΠ(Φ),where Π(Φ) is a waveform template function, for example, as shown in FIG. 1, and φ is the phase in the respiratory cycle. In FIG. 1, the waveform template is a raised cosine during the inspiratory part of the cycle, followed by a quasi-exponential decay during the expiratory portion. This shape will produce a quasi-normal and therefore comfortable flow-time curve if applied to a passive patient with normal lungs.
For example, a servo-ventilator can be constructed by setting the pressure modulation amplitude A to:A=−G∫(0.5|f|−VTGT)dt, where G is a servo gain (for example, 0.3 cmH2O per L/min per second), VTGT is a desired target ventilation (e.g., 7.5 L/min), and the integral is clipped to lie between AMIN and AMAX (for example, 3 and 20 cmH2O) chosen for comfort and safety. A servo-ventilator has the advantage of guaranteeing a specified ventilation. By setting AMIN to be non-negative, the patient can at will comfortably breathe more than the target ventilation, but in the event of a failure of central respiratory drive, the device will guarantee at least a ventilation of VTGT.
Finally, the advantages of using a waveform template can be combined with resistive unloading:5 P=P0+Rf+AΠ(Φ),whereA=−G∫(0.5|f|−VTGT)dt, 0<=AMIN<=A<=AMAX as before, giving yet more comfort to an awake patient than in the case previously considered, yet without losing a guaranteed minimum ventilation of VTGT.
A disadvantage of the pressure waveform template shown in FIG. 1 is that it is less efficient than a square wave. That is to say, for any given amplitude it provides less ventilatory support than a square wave. The waveform of FIG. 1 has only half the area of a square wave of the same amplitude. This can be a problem in patients who require a very high degree of support, or in the case of mouth leak, where in order to provide a desired pressure modulation amplitude at the glottic inlet, a much higher pressure modulation amplitude must be supplied at the mask. The use of pure resistive unloading is similarly inefficient for the same reason: the area under the pressure-vs-time curve is only half that of a square wave of the same amplitude. Even the combination of the smooth waveform template with resistive unloading is less efficient than a square wave of the same amplitude.
It is a general object of our invention to provide a pressure support ventilator that offers the advantages of using a smooth pressure waveform template while at the same time compensating for its disadvantages.
It is another object of our invention to balance comfort and effectiveness in a ventilator.